A conventional process for making electrolytic copper foil consists essentially of two steps: first, electrodeposition, or plating, of a "base" foil on a rotating drum-cathode and second, passing the foil through a "treater" machine, in order to provide the matte side of the foil with a bondable surface suitable for bonding to a polymeric substrate. The latter step is sometimes called the bonding treatment.
Traditionally, these two operations are separated by the foil manufacturers, since they seem to be mutually exclusive: formation of base foil calls for a concentrated, hot copper sulfate/sulfuric acid electrolyte, in order to yield a strong, ductile and compact deposit which forms the body of the foil, while the bonding treatment usually requires a more dilute and colder electrolyte which yields fragile, powdery deposits whose role is to enhance the true surface area of the matte side of the foil and thus enhance the bonding ability of the foil.
In a typical process, the first step, fabrication of the base foil, or "core", is primarily responsible for imparting to the bulk of the foil the combination of physical, metallurgical and electrical properties desired in the printed circuit industry, and obviously, those properties are determined by the microstructure of the bulk of the foil, which in turn is determined by purity and conditions of the plating process. Typical properties of the core of the foil sought by printed circuit board manufacturers are suitable tensile strength, yield stress, elongation, ductility and resistance to fatigue. Many of properties relate to the maximum load the material may withstand before failure, and are usually derived from stress-strain curves. Similarly, conductivity is considered an important property of copper foil. All these properties of copper foil depend on the foil's microstructure, but particularly on the microstructure of the core of the foil.
This microstructure, responsible for foil's properties, is in turn determined by the electrodeposition conditions.
Similar to other materials used in high technology applications, copper foil is a composite; i.e., it has a near-surface region with properties differing from those of the bulk material. Thus, the bulk of the copper foil (core) serves in printed circuit boards as the conductor of electricity. The matte side of the foil is responsible for promoting a permanent bond to the polymeric dielectric (insulating) substrate or prepreg, e.g., glass fabric impregnated with epoxy resin.
Metallographic cross-sectioning of copper foil reveals that the foil's two top surfaces are not the same. While the surface next to the drum, the shiny side of the foil, even when viewed under great magnification, is relatively flat and smooth, the surface next to the electrolyte, the matte side of the foil, after application of the bonding treatment is composed of extremely dense and uniform coating of spherical micro-projections which greatly enhance surface area available for bonding to the polymeric substrates.
It is to be understood that the "matte" side of the finished foil, i.e., the base foil plus treatment, refers to the combined effect of the micro-topography of the matte side surface of the base foil (electrodeposited at the drum machine) and the bonding treatment plated upon that surface at the treater machine. Both are equally important.
The cross-sections of the foils depicted in FIGS. 4(a)-(e) illustrate cross-sections of one ounce base foil for both conventional regular and low profile foils and foil made in accordance with the present invention. Copper foil has a "core" (a solid body of dense metal) and a "tooth", a chain-saw like dense coating of micro-projections composed of the micro-peaks of base foil and the bonding treatment. FIGS. 4(a) and 4(b) illustrate cross sections of a regular base foil 10 and a regular base foil 10 plus treatment 11, respectively, wherein the core 10 of the base foil has a matte surface composed of densely packed conical micro-projections 10', the R.sub.Z (average height from the peaks to the valleys) of which is typically about 400 microinches (.mu.") and the base foil plus treatment typically has an R.sub.Z of about 600.mu.". As shown in FIGS. 4(c) and 4(d), low profile base foil has a core 12 and the micro-projections 12' have an R.sub.Z typically of about 200.mu.", while the R.sub.Z of the base foil plus treatment 13 is typically about 300.mu.". FIG. 4(e) depicts foil made in accordance with the present invention which is also a low-profile foil, but the micro-projections 15 are more spherical than in the case of conventional low profile foil.
There is the question of how to determine the gauge of copper foil destined for electronic application--the weight per unit of surface area vs. the actual thickness. The former is most often used, and foil weighing one ounce per one square foot is called one ounce foil (1 oz.).
Such designation is now considered not adequate by the designers of electronic circuits and equipment, since the mass or the thickness of the "core" is pertinent in assessing the gauge (from the electrical viewpoint) of the foil, while the "tooth" is not.
Thus, it is now believed that the foil is better characterized by its thickness, measured by micrometer, since it takes into account the profile (cross-section) of the foil and the ratio between thickness of the core and the matte height, or the "tooth" (combined matte height of the foil and the treatment).
Since micrometer measurement includes peaks of the base foil and the peaks of the treatment upon them, a foil with a pronounced matte side of the base foil and with a large amount of treatment will be thicker than a foil with a less pronounced base foil matte structure and a lesser amount of the treatment, even if the weight areas of both foils are the same. The thickness of 1 oz. foil can be as different as 1.8 mil and 1.4 mil, as shown in FIGS. 4(b) and 4(d). The industry trend is toward "thinner" foil in these terms. Such foils are referred to as "low profile". Foil with a rectangular cross-section would be considered ideal, theoretically, if the foil's bonding ability were not an important consideration. However, it is widely agreed that the matte height of the foil should not exceed 15% of foil total thickness. Only such foils are used in fabrication of multi-layer boards, the most advanced and fastest growing segment of printed circuits industry.
Matte height is routinely measured by copper foil manufacturers and users, with a stylus type instrument which measures peak to valley amplitude.
Traditionally, in electroplating, the type of deposit that is best for the properties of the core of the foil is obtained from hot, concentrated electrolytes with moderate current densities. The type of deposit that is best for the properties of the matte surface or bonding surface are obtained from colder and more diluted electrolytes, with high current densities. Thus, the fabrication of the base foil and the bonding treatment are typically separated.
This, however, is a traditional view, since one can obtain very different crystalline structures in the electrodeposited copper, using the same electrolyte, by varying the other factors pertaining to the mass transfer.
In recent times, considerable advances have been made in the application of mass transfer principles to the practice of electroplating. For example, it is known that turbulent, rather than laminar, flow of the electrolytes in the anodecathode gap can increase limiting current densities, since the former can decrease diffusion-layer thickness.
U.S. Pat. No. 3,674,656 discloses a technique using a secondary anode delivering a relatively high current density to promote high-profile, high-bond matte height. While good bonding ability is achieved, the matte side of the foil, by the patent's own description is "highly roughened", in the form of "tree like growth", and would not satisfy the requirement of low-profile cross-section.
International patent No. W08703915 teaches a technique that combines agitating the electrolyte while using a secondary pulsed current having a current density greater than the limiting current density to produce copper foil having a nodularized outer surface. This technique, however, apparently does not achieve fabrication of low profile foil.
On the other hand, while low profile copper foil is currently achieved for commercial sale, it is achieved at the expense of very low production rates and/or at very low yields. It is believed that this foil production is achieved by very careful control of the more traditional process and equipment.
Such a prior art process, illustrated in FIG. 6, involves not only the separate step of treating the matte surface for bonding ability, but also a subsequent encapsulating or gilding step, a subsequent deposit of a barrier layer, typically a zinc layer, followed by a stain-proofing step (passivation) and followed by rinsing, drying and cutting. Such prior art processes are complicated and costly.